1. Field of the Invention
The present invention relates to nuclear magnetic resonance tomographs and, more particularly, to a nuclear magnetic resonance tomograph allowing to reproducibly calibrate the strength of the high-frequency magnetic field irradiated on a measuring space irrespective of the filling factor.
While the present invention is described herein with reference to a particular embodiment for a particular application, it is understood that the invention is not limited thereto. Those of ordinary skill in the art will recognize additional embodiments and applications within the scope thereof.
2. Description of the Related Art
Methods for calibrating the amplitude of a rf current of a nuclear magnetic resonance imaging apparatus are described in published international patent applications WO-A-88/09928 and WO-A-88/09929. In these prior art methods, rf test pulses are directed upon the object under test, usually a human body, and the resonance signals received are then evaluated. By selecting the pulse shape and pulse sequence of the nuclear resonance excitation pulses in a convenient manner, it is possible in this case to determine the amplitude of the rf excitation in absolute values, by evaluation of the measuring signals.
published European patent application EP-A-0 238 139 describes an image-generating nuclear magnetic resonance method where the pulse angle, i.e. the duration of the rf excitation pulses, is determined again by directing a predetermined pulse sequence upon an object under test and evaluating the measuring signals received as a response thereto.
Published European patent application EP-A-0 152 069 describes an imaging nuclear magnetic resonance apparatus where standardized reference samples are arranged in the direct neighborhood of the object under test within the measuring space. Scaling of the measuring signals received from the test objects is effected by comparing the measuring signals received from the reference samples with the measuring signals received from the object under test, i.e. a human body.
Moreover, a probe head for use in NMR tomography has been known from published German patent application DE-A-35 22 401. This probe head has substantially the form of a hollow cylinder whose outer surface and end faces are closed for rf currents and whose cylindrical inner surface is subdivided into conductive and non-conductive axial strips. Inside the known probe head a substantially homogeneous magnetic rf field is produced which has a direction perpendicular to the probe head axis.
As is generally known, an object under test, for example a portion of a human body or an entire human body, for performing nuclear magnetic resonance imaging is introduced into a measuring space which is surrounded by a conventional rf coil and also by a magnet system and, further, by gradient coils establishing constant magnetic fields having a predetermined gradient of magnetic field strength. The object under test is exposed to a constant magnetic field of high homogeneity and, further, to a rf magnetic field directed at a right angle relative to the constant magnetic field.
Whenever, in the scope of this invention, reference is made hereinafter to a "rf coil" of a "high-frequency coil", this term is to be understood as describing any high-frequency system capable of generating a high-frequency magnetic field of sufficient homogeneity and field strength in a larger three-dimensional space. Such coils may take the for of, for example, saddle coils, Helmholtz coils, line resonators, strip resonators or the like. For the purposes of the present invention, a hollow cylindrical high-frequency resonator of the type described by the afore-mentioned published German patent application DE-A-35 22 401 is particularly preferred, although the invention is by no means limited to such a probe head arrangement.
As is generally known, nuclear resonance signals are generated in nuclear magnetic resonance tomography by using pulsed high-frequency signals having a frequency proportional to the field strength of the constant magnetic field where the proportionality is given by the gyromagnetic factor of the particular nucleus under investigation.
By exciting the nuclear spins with pulsed high-frequency magnetic fields, the spin magnetization is caused to flip by a given angle relative to the direction of the constant magnetic field. This so-called flip angle is defined by the pulse area, so that the flip angle may be adjusted by adjusting the pulse length and/or the pulse amplitude.
In order to generate measuring signals of maximum intensity, i.e. optimum nuclear magnetic resonance excitation, it is customary to apply pulses having a flip angle of 90.degree. or 180.degree. in order to either flip the magnetization into a radial plane or transfer it into a state of anti-magnetization, with inverse sign.
When adjusting the nuclear magnetic resonance tomograph, one, therefore, seeks to adjust the flip angle with the greatest possible accuracy to the value of 90.degree. or 180.degree. in order to achieve the greatest possible signal yield.
Now, the determination of the pulse length does not create too big a technical problem because suitable time controls and gate circuits are available enabling the pulse duration to be adjusted with sufficient precision and at reasonable expense.
On the other hand, however, it has been mentioned before that the flip angle depends not only on the pulse length, but also on the pulse amplitude so that the latter, i.e. the amplitude of the rf magnetic field effective at the measuring space, must be adjusted as well.
Now, the amplitude of the magnetic high-frequency field is not simply proportional to the amplitude of the high-frequency excitation current within the coil. Rather, the amplitude of the magnetic high-frequency field additionally depends on the degree of loading of the high-frequency resonant circuit. If, for example, one and the same high-frequency coil is to be used for examining a very little or very thin patient on the one hand and a large or very fat patient on the other hand, then the so-called filling factor will change due to the fact that the patient's tissue leads to both dielectric losses and magnetic losses as a result of eddy currents generated the patient. In addition, the loading of the high-frequency resonance circuit may vary when the very little or thin patient, or the respective part of a patient's body does not take up the whole space within the coil and when the position of the test object in the coil is not exactly defined.
In all these cases, absolutely undefined conditions are encountered regarding the interdependence of the effective high-frequency magnetic field and the high-frequency excitation current so that it is by no means possible to achieve calibrated conditions with regard to the high-frequency fields strength by adjusting the excitation current.
It must be additionally taken into account that many countries have enacted legislations prescribing limits for the maximum permissible exposure of the human body to high-frequency radiation. In the U.S., for example, the competent FDA has promulgated standards defining a threshold value of 0.4 W of effective high-frequency power per kilogram of weight of the patient's body within the sample space.
It is, therefore, necessary not only for signal-maximizing purposes, but also in the interest of a patient's safety, to calibrate, i.e. to adjust in a reproducible manner, the effective high-frequency field strength, i.e. the amplitude of the high-frequency magnetic field.
With conventional nuclear magnetic resonance tomographs, this is achieved by initially carrying out NMR measurements with an arbitrarily adjusted amplitude of magnetic high-frequency field, with the sample space loaded, i.e. with the patient in place in the tomograph. The tomograph operator then observes the free induction decay (FID) signal on a CRT screen varying in response to the amplitude of the high-frequency magnetic field which is conventionally adjusted manually. The operator then tries--by trial and error, i.e. by varying the amplitude arbitrarily--to find the point where the FID signal reaches its maximum because a further increase of the amplitude (always related to a constant pulse length) would lead to the flip angle of, for example, 90.degree. being exceeded, and then the signal amplitude would drop again as soon as the flip angle exceeds 90.degree..
This conventional empirical method is, however, subject to a number of drawbacks:
First, this adjustment procedure is extremely time consuming as several scans have to be observed if a reliable assessment of the FID signal is to be made so that one has to wait 5 to 10 seconds, for example, per test measurement. In practice, this has the effect that at the end of this waiting period the user may have forgotten the measuring value previously adjusted; or else an impatient operator of the tomograph may not wait for the full period, but decide to repeat the measurements in quicker sequence, in which case numerous errors may slip in, for example due to dynamic effects of nuclear magnetic resonants.
In addition, considerable errors may result when the maximum of the FID signal to be found is not clearly defined. If, for example, a 180.degree. is to be determined via the maximum of the echo signal, then a corresponding maximum echo signal will be encountered also at 540.degree., i.e. generally at a flip angle equal to 2n-1 times the desired flip angle. Consequently, it may well happen that the operator of a tomograph adjusts a 540.degree. pulse instead of a 180.degree. pulse without becoming aware of his error. However, a 540.degree. pulse, as compared with a 180.degree. pulse means that the high-frequency magnetic field strength is exceeded by a factor of 3.
Erroneous adjustments of the type mentioned before are well possible in practice because the high-frequency power output of NMR tomographs has to be rated such that both very small objects under tests (small children) and very large objects under test (fat adult patients) may be examined. While in the first mentioned case, for example, a high-frequency power of 100 W would be sufficient, a high-frequency power of 2000 W may be required in the second mentioned case. Given this power reserve, there is, however, the risk that when examining a small test object a high high-frequency power and, thus, a flip angle of a higher order, with the correspondingly high and possibly even dangerous high-frequency power is set by an unexperienced or careless operator.
Now, it is an object of the present invention to improve a nuclear magnetic resonance tomograph such that the high frequency field strength can be standardized or calibrated, even for a low-frequency power, without the necessity to carry out nuclear resonance measurements, so that any damage to the patients can be definitely excluded.